There is a need for apparatus for measurement and calibration of fluid pressures of less than 1000 millitorr, and especially in the range of approximately 0-10 millitorr. There is also a need for a primary pressure standard for fluid pressures in this pressure range. The term "zero millitorr", as used herein, means a pressure of less than approximately 10.sup.-6 torr.
Devices for fluid pressure measurement at relatively low pressures are known. For example, dead-weight testers generate a pressure differential above and below a piston of variable weight and can be paired to generate relatively small differential pressures. Liquid manometers indicate pressure differences as a difference in height of a liquid in a tube or column based on the density and vapor pressure of the liquid. Volume expansion devices indicate pressure differences by controlling the expansion of a compressible fluid held at a known high pressure.
None of these devices is well suited for measuring fluid pressures in the range of 0-10 millitorr or of providing a primary pressure standard for that range of fluid pressures. The accuracy with which pressure can be measured is severely limited below 100 millitorr. Further, these devices typically measure only differential pressures, not absolute pressures. With respect to the use of liquid manometers, the properties of the liquids used, such as density and vapor pressure, vary with temperature and thus can yield unreliable measurements. It is also very difficult to measure the height of a liquid column with sufficient precision at pressures of less than about 100 millitorr Moreover, volume measurements of an expanding fluid can be difficult to make if the volumes are too small. With regard to both liquid manometers and volume expansion devices, the phenomenon of molecular adsorption of fluid on the walls of the fluid container or chamber can have a significant effect at pressures of less than 1 Torr. Contamination of the fluid is another difficulty which significantly affects pressure measurements in this range. Finally, the measurement uncertainties using these devices are relatively great, and thus the utility of such devices is limited, because they cannot provide accurate and reliable measurements in this pressure range.